While current smokers are at around 37 percent increased risk of developing Type-2 diabetes, passive smoking or breathing in second-hand smoke could raise the odds by as much as 22 percent, shows the findings.

The researchers estimated that 11.7 percent of cases of Type-2 diabetes in men and 2.4 percent in women (about 27.8 million cases in total worldwide) may be attributable to active smoking. They also found that risk decreases as time elapses after smokers quit.

"Cigarette smoking should be considered as a key modifiable risk factor for diabetes. Public health efforts to reduce smoking will have a substantial impact on the global burden of type 2 diabetes," said study co-author Frank Hu, professor of nutrition and epidemiology at Harvard TH Chan School of Public Health in Boston, US.

In this study, the researchers conducted an analysis of 88 previous studies on the association between smoking and Type-2 diabetes risk, looking at health data from nearly six million study participants. They found that when compared with people who never smoked, current smoking increased the risk of Type-2 diabetes by 37 percent, former smoking by 14 percent and passive smoking (breathing in second-hand smoke) by 22 percent.

Among current smokers, the amount smoked made a difference. The increased risk of developing Type-2 diabetes was 21 percent, 34 percent, and 57 percent for light, moderate, and heavy smokers, respectively.

The study was published in the journal The Lancet Diabetes Endocrinology.